Scala 对重载方法的类型推断

Question

鉴于此代码:

class Rational(n: Int, d: Int) {
  require(d != 0)
  private val g = gcd(n.abs, d.abs)
  val numerator = n / g
  val denominator = d / g

  def this(n: Int) = this(n, 1)

  override def toString = numerator + "/" + denominator

  def +(r: Rational) = new Rational(numerator * r.denominator + r.numerator * denominator, denominator * r.denominator)

  def *(r: Rational) = new Rational(numerator * r.numerator, denominator * r.denominator)

  def +(i: Int) = new Rational(i) + this

  private def gcd(a: Int, b: Int) : Int = {
    if (b == 0) a else gcd(b, a % b)
  }

}

为什么 Scala 不能推断出 +(i: Int) 返回一个有理数？ (fsc 给出 overloaded method + needs result type 错误)

如果我将该方法更改为:

def +(i: Int): Rational = { new Rational(i) + this }

有用...

Answer 1

我在 Scala 邮件列表中发现了一个主题完全相同的问题 here .那里的答案解释了为什么需要提供返回类型。进一步调查后，我还发现了这个:When is a return type required for methods in Scala .如果我应该引用那里的答案:

When Explicit Type Annotations Are Required.

In practical terms, you have to provide explicit type annotations for the following situations:

Method return values in the following cases:

• When you explicitly call return in a method (even at the end).
• When a method is recursive.
• When a method is overloaded and one of the methods calls another. The calling method needs a return type annotation.
• When the inferred return type would be more general than you intended, e.g., Any.